1. Technical Field
The present invention relates to semiconductor wafer processing. More particularly, the present invention relates to securing a semiconductor wafer to a wafer pedestal during processing.
2. Description of the Prior Art
It is important to maintain a nearly constant temperature across the surface of a semiconductor wafer during wafer processing, for example during plasma etching in a reaction chamber. Temperature uniformity is a critical part of assuring process reproducibility, i.e. achieving consistent processing results, as shown by high device yields and low device defect densities. It is also important to avoid subjecting the wafer to excessive temperatures that could damage the delicate features of the devices formed on the wafer surface.
An inert gas, such as Helium or Argon, is commonly used during wafer processing as a thermal transfer medium in the gap that exists between the wafer backside and a wafer pedestal to which the wafer is clamped, to thereby take advantage of the large thermal mass of the pedestal relative to the wafer. In this way, a consistent and predictable temperature is maintained across the wafer surface during wafer processing, and the various process steps used to fabricate devices on the wafer surface may be carried out in a reliable fashion.
Two shapes of wafers are commonly used in the semiconductor industry: round wafers, and wafers having a flat peripheral portion. Round wafers are aligned during wafer processing by means of a small notch placed in the wafer edge, while wafers having a flat portion are aligned on the wafer flat portion.
When processing a round wafer, a conventional wafer clamping arrangement, such as is shown in FIG. 1, secures the wafer 10 to a wafer cooling pedestal 12 with an annular wafer clamping ring 11. The clamping ring is used to press the edge of the wafer into a continouous sealing abutment with upper surface of the wafer pedestal. A port 13 is provided to flow a supply of coolant gas 14, such as Helium, to the backside of the wafer to improve thermal transfer between the wafer and the pedestal, as discussed above.
It is well recognized in the industry that leakage of cooling gasses, such as Helium, into the process environment from the backside region of the wafer is detrimental to the fine chemical balance required for wafer processing. Morever, a poor seal between the wafer and the wafer pedestal also allows reactants in the process environment to penetrate the regions at the backside of the wafer, where they can contaminate and/or etch the wafer backside and the wafer pedestal.
The seal between the wafer edge and the pedestal created by application of clamping pressure to the wafer edges prevents leakage of excessive amounts of cooling gas from the backside of the wafer into the wafer process environment, thereby eliminating or significantly mitigating cooling gas interference with critical wafer processing parameters and chemistry, and also restricting the possibility of volatile process components reacting with the backside of the wafer and the wafer pedestal.
Leakage of cooling gas applied to the backside of a wafer into the process environment is a daunting problem when wafers having a flat portion are processed because a discontinuous seal is formed between the wafer pedestal and the edge of the wafer that creates a gas leak gate. That is, it is not possible to secure a wafer, such as the wafer 20 shown in FIG. 2, having a flat portion 24 to a wafer pedestal with a conventional clamp ring to form a continuous seal between the edges of the wafer and the pedestal that prevents an excessive flow of cooling gas into the process environment.
FIG. 3a is a side elevation view showing the exposed flat portion 24 of a semiconductor wafer 20 when the wafer is secured to a wafer pedestal 12 by a prior art wafer clamping ring 26. A gap 22 exists at the point where the flat portion of the wafer coincides with the pedestal surface that allows relatively unrestricted flow of cooling gas from the wafer backside into the process environment.
The clamp ring 26 may be used to apply clamping pressure over the wafer flat portion, as well as the round portion of the wafer. See, for example FIG. 3b, which is a section taken along line A--A of the clamping arrangement shown in FIG. 3a. In the figure, the wafer clamp 26 contacts both the round portion of the wafer at the wafer edge 25, and the flat portion of the wafer at the flat edge 24. The clamp contacts each portion of the wafer and presses the wafer edges into abutment with the wafer pedestal at discrete locations on the wafer pedestal having different radii, such that clamping pressure is not distributed evenly and continuously about the edge of the wafer. As a result, cooling gas leaks from the backside of the wafer into the process environment.
Thus, while prior art clamp rings, in combination with a conforming pedestal wafer clamping area, distribute adequate pressure evenly about the edges of a wafer to provide a continuous seal on round wafers, such clamping arrangement cannot satisfactorily seal the edges of a wafer having a flat portion.
It is known to seal the edges of a round wafer to a pedestal with an elastomeric seal to prevent coolant gas leakage. See, for example Studley et al, Perimeter Wafer Seal, U.S. Pat. No. 4,923,358 (12 Jun. 1990). Using an elastomeric seal in a process chamber is unsatisfactory because the elastomer generates particles that can contaminate the chamber and wafers processed in the chamber; it limits the possibility of plasma cleaning because the reactant gases used to accomplish such cleaning may react with the elastomer; and it prevents wafer processing at the higher temperatures typically encountered in a process environment, which range up to about 400.degree. C., because the elastomers melt at high temperatures. Additionally, Studley et al requires the use of an elaborate mechanism to press the clamp ring to the pedestal. The clamping mechanism applies sufficient clamping pressure to the wafer when forcing the wafer into abutment with the pedestal that the wafer is stressed and may even be broken. A clamping ring adapted to secure a wafer having a flat portion to a wafer pedestal is disclosed in Sherstinsky et al, Clamping Ring Apparatus For Processing Semiconductor Wafers, U.S. patent application Ser. No. 07/947,212 (filed 18 Sep. 1992 and commonly assigned to Applied Materials, Inc.) now U.S. Pat. No. 5,316,278. The disclosed clamping ring provides a plurality of yieldable metallic fingers that project radially inwardly, and that are spaced circumferentially about the clamp ring, to compensate for any discrepancy in wafer height and/or shape, for example due to variations in clamping radius resulting from clamping the flat portion of the wafer instead of the round portion of the wafer.
While such clamp ring is useful to mitigate the problem of cooling gas leakage from the backside of the wafer and the pedestal for many applications, it is not well adapted for applications where the exposed metallic fingers are likely to react with process chemicals, for example as encountered during plasma etching. The clamp ring is therefore limited to nonvolatile processes involving reactants, or gases that cannot either damage the clamp ring, for example by etching or deposition; or interact with the materials of the clamp ring to produce byproducts that can poison the process. Another concern is that it is more difficult to manufacture a clamp ring having a circumferential array of radially projecting fingers than a clamping ring having a continuous flat clamping surface.
It would therefore be useful to improve the art of wafer clamping to secure wafers having a flat portion to a wafer pedestal such that cooling gases cannot leak from between the backside of the wafer and the wafer pedestal during wafer processing in all processing environments and under all processing conditions.